1. Field of the Invention
The present invention concerns an arrangement to correct eddy currents in a gradient coil of a magnetic resonance apparatus.
2. Description of the Prior Art
Magnetic resonance apparatuses are known that have actively shielded gradient coils. A gradient coil is associated with each spatial direction, i.e., with each spatial coordinate axis. The three gradient coils form a gradient coil system.
The gradient coil system normally is surrounded by conductive structures, in which the eddy currents are induced by switched gradient fields. The magnetic fields generated by the eddy currents attenuate and distort the time curve of the desired gradient field. Actively shielded gradient coils are used in order to largely compensate for the field distortions. For example, a shielding coil associated with a gradient coil has a lower winding count and is associated with the gradient coil such that the same current magnitude that flows through the gradient coil also flows through the shielding coil, but in the opposite direction.
Due to the connective wiring, the shielding coil is generally designated as a secondary gradient sub-coil while the field-generating gradient sub-coil that is actually required for magnetic resonance imaging is designated as a primary gradient coil.
For these gradient coils it is absolutely necessary to limit or to compensate for structural tolerances that occur in production, since these would lead to deviations of the desired magnetic field curve.
Tolerances in the structure are caused by shifts of conductor trace layers or of conductor trace packets in the axial direction, in the azimuthal direction and in the radial direction.
Unwanted couplings between the gradient coils are caused by the tolerances in the structure, these couplings in turn leading to changes of the inductance of the gradient coil and to changes in the sensitivity.
In particular, shielding errors between the primary gradient coil and the secondary gradient coil are critical, since these become noticeably disruptive in the imaging.
To address the manufacturing tolerances, gradient coils are designed in the framework of a precision production wherein mechanical tools and spacers are used in order to exactly arrange the conductor traces. With such mechanical means it is possible to satisfy the tolerance requirements of the gradient coil system for a magnetic resonance apparatus with a 60 cm patient bore.
Newer magnetic resonance apparatuses with a 70 cm patient bore have a gradient coil with a smaller thickness and therefore a higher sensitivity to mechanical tolerances. Due to the smaller gradient coil thickness, gradient coils of such a design are particularly prone to installation (mounting) deviations that lead to disruptive eddy current influences.